The computer "operated" by means of pencil and sliding cards. Any arithmetic was done in the head of the person operating the computer. The computer operated in base 10 and had 100 memory cells which could hold signed numbers from ±0 to ±999. It had an instruction set of 10 instructions which allowed CARDIAC to add, subtract, test, shift, input, output and jump.

Hardware

The “CPU” of the computer consisted of 4 slides that moved various numbers and arrows to have the flow of the real CPU (the user's brain) move the right way. They had one flag (+/-), affected by the result in the accumulator.

Memory consisted of the other half of the cardboard cutout. There were 100 cells. Cell 0 was “ROM”, always containing a numeric "1"; cells 1 to 98 were “RAM”; available for instructions and data; and cell 99 could best be described as “EEPROM”.

Memory cells held signed decimal numbers from ±0 to ±999 and were written with a pencil. Cells were erased with an eraser. A “bug” was provided to act as a program counter, and was placed in a hole beside the current memory cell.

Programming

CARDIAC had a 10 instruction machine language. An instruction consisted of three decimal digits (the sign is ignored). The first digit was the op code (O), the second and third digits was an address (A). Addressing was one of accumulator to memory absolute, absolute memory to accumulator, input to absolute memory and absolute memory to output.

OAA

High level languages were never developed for CARDIAC, since they would defeat one of the purposes of the device, to teach a little assembly language programming.

Programs were hand assembled, then written, by pencil into the appropriate memory cells.

Instruction Set

Opcode

Mnemonic

Description

0

INP

Input – take a number from the input card and put it in a specified memory cell.

1

CLA

Clear and add – clear the accumulator and add the contents of a memory cell to the accumulator.

2

ADD

Add - add the contents of a memory cell to the accumulator.

3

TAC

Test accumulator contents – performs a sign test on the contents of the accumulator.

4

SFT

Shift – shifts the accumulator x places left, then y places right, where x is the upper address digit and y is the lower.

5

OUT

Output – take a number from the specified memory cell and write it on the output card.

6

STO

Store – copy the contents of the accumulator into a specified memory cell.

7

SUB

Subtract – subtract the contents of a specified memory cell from the accumulator.

8

JMP

Jump - jump to a specified memory cell. The current cell number is written in cell 99. This allows for one level of subroutines.

9

HRS

Halt and reset – move bug to the specified cell, then stop program execution.

Operation

Programs were run by first sliding three slides so that the number in the instruction register equaled the number in the memory cell the bug was sitting in. Once that was done the bug was moved to the next memory cell. The user then followed an arrow which would then tell them what to do next. This would continue for all of program execution.

Implementations

There are several ways to get one of these devices. One way is from Comspace Corporation at Another way is a recreation based on pictures from the internet here Finally, a simulator for the Java platform, called jcinc, with both command-line and GUI interfaces, is at Sourceforge